PNAS First Look Blog

Science journalists discuss a selection of new papers from PNAS

Detecting life on exoplanets harder than thought

Astronomers have confirmed the existence of more than 1,000 worlds outside our solar system, potentially increasing the prospects of detecting signs of life on an exoplanet. However, scientists now propose a potential source of confusion that could dampen hopes of conclusively discovering signs of alien life. The findings are detailed in the Proceedings of the National Academy of Sciences.

A strategy often suggested for searching for signs of life on exoplanets involves looking for ways that life might change a world’s appearance, such as how key chemicals might change the spectrum of light seen from those planets. These searches focus on biomarkers and biosignatures — chemicals or combinations of chemicals that could be produced by life and that could not be or are unlikely to be produced abiotically by processes other than life.

One of the most promising biosignatures is the combined presence of two molecules that ordinarily would destroy each other in the atmosphere, such as oxygen and methane. Such an extreme “disequilibrium” might suggest that life was present to generate large amounts of either or both of the molecules.

However astrophysicist and planetary scientist Hanno Rein at the University of Toronto and his colleagues now reveal a scenario where the combined light of a lifeless exoplanet and its lifeless moon could lead to a seeming case of disequilibrium suggestive of life.

The researchers explored what might happen if both an exoplanet and its moon possessed its own atmosphere. After simulating an exoplanet with an oxygen-rich atmosphere such as Earth’s and a moon with a methane-rich atmosphere such as Titan’s, the scientists found the light from the exoplanet would look as if it came from a single atmosphere with a strong disequilibrium — the light of the moon blended with and contaminated the light from the exoplanet.

Rein and his colleagues noted it would be extremely difficult to discern that an exoplanet even had a moon. Moreover, the ability to detect whether an exoplanet’s moon possessed an atmosphere that might lead to a false biosignature might be unattainable with telescopes available in the foreseeable future.

“Personally, I was surprised that an unambiguous detection of life on Earth-like planets around sun-like stars is well beyond our reach for the foreseeable future,” Rein says. “On the positive side, if we relax the requirement of looking for life on a planet around sun-like stars and go to fainter stars, we might just be able to do it.”

Categories: Astronomy
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